The invention relates to a process for forming an initial section or like component featuring a hollow interior to a final shape by means of high internal pressure in the sealed hollow interior using a medium that can flow, in particular forming until the final section comes into contact with the wall of a shape-determining space. Further, the invention also relates to a section with a space delimited by section walls, in which two section walls each determine a corner region of the cross-section, in particular an initial section for carrying out this process.
In the high-internal-pressure-forming (HIPF) process a hollow section is expanded by means of internal pressure. In addition, by means of at least one stem engaging the part in question, the hollow section may be displaced and widened, compressed or expanded.
German Patent DE 35 32 499 C1 describes a device for hydraulic expansion of a length of pipe by using a plug-like cylindrical probe which can be introduced into the pipe and, using at least a pair of sealing rings spaced a distance apart, forms a circular space which is filled with compressive medium for the purpose of expanding the tube. Each of the sealing rings is situated in a ring-shaped groove with a u-shaped in cross-section, in the probe and initially upon introducing the probe into the tube, has an outer diameter which at most is the same as the outer diameter of the probe. Before starting the expansion process, in order to seal the ring-shaped gap between the probe and the tube, compressive medium is introduced into the ring-shaped grooves via a feed pipe connected to the medium supply line and applies compressive force radially to the sealing rings. The feeding of the compressive medium to the ring-shaped space is performed solely by way of at least one of the grooves and is controlled by a sealing ring acting as a valve, which closes off an opening between the groove and the ring-shaped space until it has achieved its sealing function by elastic expansion. That groove is provided with at least one inclined slit at its edge neighboring the ring-shaped space. If the pressure in the ring-shaped space between the two seals is increased, the wall of the tube begins to expand in this region.
This internal high pressure forming or hydroforming process is finding ever increasing application in the automobile industry as an economical means for manufacturing car body components. Mainly steel tubes are employed as starting material. The final contour of the component to be shaped this way is generally much more complicated than the simple circular cross-section of the starting material. As a rule, the HIPF process results in regions which are much more heavily deformed than other regions and which are correspondingly thinner. If these regions are subjected to a high degree of loading in use, the initial sheet must be sufficiently thick; this however results in an unnecessary amount of material in the less heavily formed regions. This disadvantage is contrary to the requirement of obtaining the lowest possible weight in the component.
Recently aluminium alloys have been included along with steel as starting material for HIPF processes. As with steel there are manufacturing processes in which tubes of aluminium sheet are employed as starting material; alternatively, extruded aluminium sections may also be employed for that purpose. For economic reasons extruded steel sections do not come into question here. The use of extruded sections has the decisive advantage that the shape of the initial section is almost without limit.
HIPF processes using extruded sections are employed mainly to be able to produce high precision parts. To that end the present state of the art tends to make the shape of the initial section as close as possible to that of the final section in order to employ relatively small degrees of deformation in the HIPF-process. In particular with curved components that are to be bent in advance or where the section cross-sections feature sharp corners, this approach is usually not successful. Also, attempts to keep the degree of deformation small generally results in its non-uniform distribution. As a result—and due to the pre-shaping from the bending process—spring-back effects are produced causing the desired precision to be achieved only in exceptional cases using that process. Likewise as a rule, sharp corners which exhibit a large ratio of wall thickness to outer radius cannot be filled out using this process.
In HIPF-processes using steel pipes it is normal to carry out pre-shaping prior to the actual shaping process (bending and HIPF)—this e.g. in order to arrive at a more favourable cross-section for bending or in order to make it even possible to place the part in the HIPF shaping tool.